Permanent magnets create their own persistent magnetic fields and are typically used in electromagnetic induction devices such as motors and generators. Permanent magnets are made from a magnetic material such as ferrite. Additional magnetic materials from rare earth metals, such as Samarium-Cobalt (SmCo), or Neodymium-Iron-Boron (NdFeB) have also been used. While an NdFeB magnet is stronger (e.g., according to maximum energy product (BH)max) than an SmCo magnet which is stronger than ferrite, it becomes difficult to increase the magnetic strength even further and Neodymium magnets are much more expensive than ferrite due to the scarcity of Neodymium.
Moreover, a magnet is typically strongest at the surface of the magnet. The magnetization at the magnet surface then decreases with distance from the magnet surface due to a large self-demagnetization field generated by surface magnetic charges, thereby reducing the strength of the magnet away from the surface, such that the magnetic flux density decreases with distance from the magnet. Other various magnetic properties such as magnetic anisotropy, magnetic moment and thus magnetic flux density, etc. may decline as the operating temperature increases. Additionally, there are several grades of NdFeB, SmCo, ferrite, etc. magnets, where higher grades indicate stronger magnets. However, the cost of the magnet may increase in proportion with the grade.
The torque generated in an electromagnetic induction device, such as a motor is proportional to the magnetic flux density produced by a permanent magnet in the stator. As the magnetic flux density increases, the efficiency of the motor increases. This also applies to generators where the electric current and thus the generator efficiency increases in proportion to the magnetic flux density.